The invention relates to a twin-flow beater mill which enables fibrous materials to be prepared by disintegration parallel to the fibers into intermediate products such as are required for further industrial processing, for example, in the board or pulp industries.
Such fibrous materials occur as waste products, for example in the wood-processing and wood-working industries in the form of sawdust and planing shavings. However, large amounts of fibrous materials are also produced in the processing of annual plants, such as, for example, in the sugar-cane industry with regard to the so-called bagasse. Large amounts of fibrous waste materials are also to be prepared using grinding technology in the reuse of old paper.
Economic considerations dictate that the preparation of these fibrous materials to produce intermediate products capable of further processing be performed at high rates of throughput and with a low specific energy requirement. These operational conditions are fulfilled in principle by the so-called twin-flow beater mill in which the charging is performed in the axial center of a cylindrical grinding surface, from where the flow of material passes outwards through the annular grinding gap formed between the active edges of the beater plates and the grinding surface on two symmetrically axially opposed helical surfaces, aided by the air flow caused by the beater rotor. This mode of operation yields a high rate of throughput in conjunction with optimum utilization of the entire grinding surface area. In addition, it is possible by the purposeful selection of design parameters, such as grinding surface configuration, grinding gap width, number of beater plates and the like, or else by the selection of suitable operational parameters, such as speed of the beater rotor or influencing of the air flow, to conduct the grinding process in such a way that the ground material is subjected only to as much energy as is just sufficient for the targeted degree of comminution of the respective material.
Such a twin-flow beater mill has been disclosed in German Patent 1,909,022. It has a beater rotor, which is fitted with beater plates and surrounded concentrically by a cylindrical grinding surface, and whose rotor disks which carry the beater plates in the axial rotor center form an axially charged guide duct shaped like an annular disk which opens out peripherally onto the grinding surface. Correspondingly cylindrical screen webs which determine the targeted degree of fineness of the ground material are arranged on both sides of the grinding surface.
Although this type of mill has proved outstanding in the comminution of numerous types of material, substantial problems occur, especially with a damp charge, in processing fibrous materials which contain fractions that are overlong and, additionally, thin, that is to say in the form of strands or strings. This enables the use of this type of mill in the special industrial field of the preparation of fibrous materials only with additional complicated measures, if at all.
Thus, in the known twin-flow beater mill, the rear edges of the beater plates, which bridge the guide duct, which is shaped like an annular disk in its peripheral opening-out region, act as a trap for the fibrous fractions in the form of strands. As a result, it is possible, especially with a damp charge, for them to build up on the inside of the beater plates irregular accumulations of material which cause eccentric unbalanced masses, so-called unbalance, on the beater rotor, the consequence of which is uneven running of the machine. Moreover, in this way the beater rotor clogs up gradually and over its circumference in an irregular distribution, resulting in a pulsating flow of material in conjunction with a decreasing rate of throughput. The known twin-flow beater mill has therefore had to be frequently shutdown for the purpose of scraping the beater rotor. However, even the installation of special scrapers has not been able to provide a satisfactory remedy here, despite a substantial outlay on design.
In addition, with the known twin-flow beater mill it is not possible for the degree of disintegration of the fibrous materials to be quickly adapted by simple measures to changed operating conditions such as can be caused in the event of a change in state of the charge or as a consequence of changed requirements for further processing. Because of the risk of blockage, it is not possible to use the replaceable screen rings that are arranged on both sides of the grinding surface and which have proved themselves for determining the degree of fineness in the case of granular ground material for fibrous materials. As a result, it was necessary for the retention time of the material on the grinding surface, and thus its degree of disintegration, to be influenced only by exchanging the grinding surface for one having a different angular orientation of the grinding surface ribs. Not only was this removal expensive and the conversion time consuming, but it also required the service personnel to have a high degree of practical knowledge.